Optical pickup-use object lens, optical pickup and optical disk unit

ABSTRACT

The present invention is relative to an objective lens ( 15 ) for an optical pickup, having a numerical aperture not less than 0.8, and adapted for correcting the chromatic aberration at an image point on an optical axis for light with a wavelength within several nm about a reference wavelength which is not larger than 420 nm. This objective lens is made up by a first lens set GR 1 , having a compound surface S 2  of a refractive surface S 2r  and a diffractive surface S 2d , and a second lens set GR 2 , having a positive refractive power, looking from the light source side. The compound surface of the first lens set is formed by adding the diffractive surface having the positive refractive power to the refractive surface which is an aspherical surface having the negative refractive power. The lens forming the second lens set is a single lens including at least one aspherical lens. This objective lens is small-sized, permits effective correction of chromatic aberration and enables the laser light to be converged to close to the diffraction threshold.

TECHNICAL FIELD

[0001] This invention relates to an objective lens used in illuminatinga light beam, such as a laser light beam, for recording the informationand for reproducing the information recorded on the optical recordingmedium, an optical pickup device employing this objective lens, and toan optical disc device. More particularly, it relates to an objectivelens capable of converging a light beam to a diffraction limit on arecording surface of an optical recording medium, to an optical pickupdevice employing this objective lens, and to an optical disc device.

BACKGROUND ART

[0002] As an information recording medium, a non-contact type opticalrecording medium, having a high recording density, is now in use. Theinformation is recorded on the optical recording medium by illuminatinga light beam, such as a laser light beam, on the recording surface,while the information recorded on the optical recording medium may bereproduced in similar manner. As this sort of the optical recordingmedium, a recording medium formed an optical disc, that is a disc-shapedoptical recording medium, is finding widespread use because of ease inretrieving the recorded information.

[0003] An optical disc has a spirally or concentrically formed recordingtrack(s). The distance between center lines of neighboring recordingtrack turns, that is the track pitch, is approximately 1.6 μm in thecase of for example a CD (Compact Disc), while being smaller and 0.74 μmin the case of a DVD (Digital Versatile Disc), for thereby improving theinformation recording density appreciably.

[0004] For illuminating the laser light on an optical disc where thetrack pitch on the recording tracks of a DVD, etc. is decreased toimprove the information recording density, a beam spot smaller than inthe case of an optical disc with a larger track pitch needs to be formedon the recording surface of the optical disc.

[0005] The diameter of the beam spot of the laser light beam, convergedby the objective lens, is proportionate to the design wavelength of thelaser light, while being inversely proportionate to the numericalaperture (NA) of the objective lens. Thus, for reducing the beam spotdiameter, it is necessary to increase the numerical aperture of theobjective lens and to decrease the laser light wavelength.

[0006] For recording the information on an optical disc by a phasechange method, or still other methods, the laser light of high lightenergy is required. On the other hand, the laser light noise due toreflected laser light needs to be reduced. For this reason, the drivingpower is varied by such methods as by superposing high frequency on thedriving current or voltage of the semiconductor laser to vary thewavelength of the laser light in a shorter period. Consequently, in anoptical pickup designed for illuminating a coherent laser light beam onthe optical disc, chromatic aberration ascribable to wavelengthvariations of several nm tends to be produced, thus increasing the sizeof the beam spot on the optical disc.

[0007] An optical pickup includes an objective lens for converging alaser light beam 200 on the recording surface of an optical disc. Astate-of-the-art objective lens 201, shown in FIG. 1, is formed by asole lens, obtained on glass molding, and has an aspherical surface 201a of high light converging performance.

[0008] As may be seen from the graphs of the spherical aberration ofFIG. 2A, astigmatic aberration of FIG. 2B and the distortion aberrationof FIG. 2C, the chromatic aberration of the order of ±0.6 μm/nm isgenerated with this objective lens 201 for the wavelength variation ofthe order of +0.2 nm, even with the use of an aspherical surface 201 a.It should be noted that, in the aberration diagrams of FIGS. 2A to 2C,solid lines, broken lines and chain-dotted lines indicate the values ofthe aberration at 405 nm, 403 nm and 407 nm, respectively, and that, inFIG. 2B, showing astigmatic aberration, thick lines and fine linesindicate values in a sagittal image surface and in a tangential imagesurface, respectively.

[0009] For recording the information on an optical disc, in which highrecording density is achieved by narrowing the track pitch of therecording medium, it is desirable to converge the laser light to closeto the diffraction threshold by an objective lens to form a smaller beamspot. However, with the state-of-the-art monolithic objective lens 201,obtained on molding the glass having the aspherical surface 201 a, it isdifficult to converge the laser light to close to the diffractionthreshold, because of generation of the chromatic aberration, asdescribed above.

DISCLOSURE OF THE INVENTION

[0010] The present invention has been proposed in view of the state ofthe art, described above, and aims to provide a novel objective lensusable with advantage for an optical pickup used for recording and/orreproducing the information for an optical recording medium in which thetrack pitch of the recording track is narrowed to increase the recordingdensity. The present invention also aims to provide a small-sizedobjective lens which is able to compensate for chromatic aberrationeffectively and to converge the light beam to close to a diffractionthreshold.

[0011] Moreover, the present invention aims to provide an optical pickupemploying an objective lens capable of effectively correcting thechromatic aberration and of converging the light beam to close to adiffraction threshold, and an optical disc employing this opticalpickup.

[0012] For accomplishing the above objects, the present inventionprovides an objective lens for an optical pickup, having a numericalaperture not less than 0.8, and adapted for correcting the chromaticaberration at an image point on an optical axis for light with awavelength within several nm about a reference wavelength which is notlarger than 420 nm, wherein the first lens set includes a compoundsurface constituted by adding a diffractive surface having a positiverefractive power to an aspherical refractive surface having a positiverefractive power, an amount of sag of the aspherical surface of thefirst lens set having a negative refractive power is described by apolynominal of an even order number with respect to a radius, as thecone coefficient of the aspherical coefficient (k) is set to −1, anamount of sag of the diffractive surface of the first lens set isdescribed by a polynominal of an even order number with respect to aradius, the order number of the polynominal of the aspherical surface isequal to the order number of the polynominal of the diffractive surface,and the coefficients of the same order numbers of the polynominal of thesag of the aspherical surface are equated to those of the polynominal ofthe sag of the diffractive surface in such a manner as to meet theequations:

k=−1

C ₁=(N−1)c/2

C ₂=(N−1)A

C ₃=(N−1)C

C ₄=(N−1)D

[0013] where C₁, C₂, C₃, C₄, . . . are coefficients of respective ordernumbers of the polynominal of the aspherical surface, c/2 is the secondorder coefficient of the polynominal of the diffractive surface and A,B, C, D, . . . are coefficients of the respective orders of thepolynominal of the diffractive surface, with the lens forming the secondlens set being formed by a single lens including at least one asphericalsurface.

[0014] This objective lens effectively corrects the chromatic aberrationfor a wavelength range of several nm centered about a referencewavelength of 420 nm to enable the light beam spot diameter to bewine-pressed to close to the diffraction threshold. In an optical pickupemploying this objective lens, and with an optical disc device,employing this optical pickup, it is possible to record the informationto a high density on an optical recording medium in which the trackpitch of the recording tracks is narrowed to allow for high densityinformation recording, while it is also possible to reproduce theinformation correctly from the recording medium in which the informationhas been recorded to a high density.

[0015] With the objective lens of the present invention, in which theaspherical coefficients of the diffractive surface of the first lens setand the aspherical coefficients of the aspherical refractive surface,with the negative refractive power, to which is added the diffractivesurface, cancel each other at the same order numbers, the diffractivesurface can be formed to a staircase-like cross-sectional shape, inwhich it is possible to eliminate e.g., non-machinable portions whichtend to deteriorate the diffraction efficiency or undulatingirregularities which tend to deteriorate the transmittance in moldingthe diffractive surface during molding of the diffractive surface.

[0016] Since the first lens set of the objective lens has thediffractive surface the cross-sectional shape of which is the staircaseshape having step differences which will give the phase differencesequal to integer number multiples of the design wavelength, it ispossible to eliminate, in molding the diffractive surface, e.g.,non-machinable portions which tend to deteriorate the diffractionefficiency or saw-tooth like undulations which tend to deteriorate thetransmittance.

[0017] By employing the objective lens, including the first and secondlens sets, the component lenses of which are arranged in a common lensbarrel, for an optical pickup, it is possible to improve the workingperformance in assembling the optical pickup.

[0018] With the objective lens of the present invention, in which thefirst lens set has its surface closest to the light source side, as aplanar surface, and has a zero refractive power with respect to thedesign wavelength, the first lens set can be used only for correctingthe chromatic aberration, while the second lens set can be used only forconverging the laser light, thus assuring facilitated designing of therespective lens sets.

[0019] Since the lenses forming the first lens set is formed ofsynthetic resin, the objective lens in its entirety can be reduced inweight to reduce the production costs.

[0020] Since a protective cover, 0.3 mm or less in thickness, isprovided between the second lens set and the image surface, and thespherical aberration chargeable to the protective cover is corrected, itis possible to eliminate the effect ascribable to the sphericalaberration produced by the protective cover.

[0021] Since the lenses forming the first lens set and the lens barrelare formed as one from synthetic resin, it becomes possible to reducethe number of component parts and the weight of the objective lensitself as well as to diminish the production error in lens assembling,so that it becomes possible to reduce the weight of the objective lensto stabilize its performance as well as to reduce the cost.

[0022] Moreover, since the aperture provided between the first andsecond lens sets is formed by a thin film of e.g., metal, provided onthe surface towards the light source side of the refractive type lensforming the second lens set, it becomes possible to suppress theproduction error in assembling the lens to stabilize its performance.

[0023] Additionally, since the concentric irregularities finer thanirregularities of the diffractive surface are formed on the compoundsurface of the first lens set, the concentric irregularities being of aperiodic structure having a period equal to about one fourth thereference wavelength and having an amplitude equal to approximately onehalf the reference wavelength, it becomes possible to elevate thetransmittance of light of a constant wavelength incident on theobjective lens.

[0024] Other objects, features and advantages of the present inventionwill become more apparent from reading the embodiments of the presentinvention as shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a side view showing an objective lens used in astate-of-the-art optical pickup.

[0026]FIG. 2A is a graph showing the spherical aberration of theobjective lens shown in FIG. 1, FIG. 2B is a graph showing itsastigmatic aberration and FIG. 2C is a graph showing its distortionaberration.

[0027]FIG. 3 is a schematic perspective view showing the appearance ofan optical disc device to which the present invention is applied.

[0028]FIG. 4 is an exploded perspective view showing an inner structureof the optical pickup device.

[0029]FIG. 5 is a side view showing an optical pickup embodying thepresent invention.

[0030]FIG. 6 is a graph showing the relation between the radial lengthof the objective lens for the optical pickup of the present inventionand the sag in a diffraction surface and in both a diffraction surfaceand a refraction surface.

[0031]FIG. 7 is a graph showing the relation between the radial lengthof the objective lenses for the optical pickup of the present inventionand the sag in a compound surface comprised of the combination of thediffraction surface and a refraction surface.

[0032]FIG. 8 is a schematic perspective view for illustrating theformulation of a blade mold diffraction surface by machining a transfermetal mold employing a diamond byte.

[0033]FIG. 9 is a schematic perspective view for illustration offormulation of a staircase type diffraction surface by machining atransfer metal mold employing a diamond byte.

[0034]FIG. 10 is a graph showing the relation between the width of anon-machined portion left in forming the blade mold diffraction surfaceby machining a transfer metal mold employing a diamond byte.

[0035]FIG. 11 is a schematic longitudinal cross-sectional view showingthe shape of a blade mold diffraction surface when the width of thedistal end of the diamond byte cannot be disregarded.

[0036]FIG. 12 is a graph showing the relation between the height of theundulating irregularities at the time of preparation of the blade typediffraction surface by the machining processing of a transcription metalmold employing a diamond byte.

[0037]FIG. 13 is a schematic longitudinal cross-sectional view showingthe shape of undulating irregularities at the time of preparation of theblade type diffraction surface by the machining processing of atranscription metal mold employing the diamond byte.

[0038]FIG. 14 shows the state of incident light and outgoing light on asurface presenting undulating irregularities.

[0039]FIG. 15 is a graph showing the relation between the amplitude ofthe structure of irregularities in a diffraction surface used with redlaser light and transmittance.

[0040]FIG. 16 is a graph showing the relation between the amplitude ofthe structure of irregularities in a diffraction surface used with bluelaser light and transmittance.

[0041]FIG. 17 is a side view showing another embodiment of an opticalpickup embodying the present invention.

[0042]FIG. 18 is a side view showing another embodiment of an objectivelens for an optical pickup embodying the present invention.

[0043]FIG. 19 is a longitudinal view showing a further embodiment of anobjective lens for an optical pickup embodying the present invention.

[0044]FIG. 20 shows a lens structure of a first embodiment of theobjective lens for an optical pickup according to the present invention.

[0045]FIG. 21A is a graph showing spherical aberration of the firstembodiment of the objective lens, FIG. 21B is a graph showing astigmaticaberration thereof and FIG. 21C is a graph showing distortion aberrationthereof.

[0046]FIG. 22 shows a lens structure of a lens of a second embodiment ofan objective lens for an optical pickup according to the presentinvention.

[0047]FIG. 23A shows a graph showing spherical aberration of the secondembodiment of the objective lens, FIG. 23B is a graph showing astigmaticaberration thereof and FIG. 23C is a graph showing distortion aberrationthereof.

[0048]FIG. 24 shows a lens structure of a third embodiment of anobjective lens for an optical pickup according to the present invention.

[0049]FIG. 25A shows a graph showing spherical aberration of the thirdembodiment of the objective lens, FIG. 25B is a graph showing astigmaticaberration thereof and FIG. 25C is a graph showing distortion aberrationthereof.

[0050]FIG. 26 shows a lens structure of a fourth embodiment of anobjective lens for an optical pickup according to the present invention.

[0051]FIG. 27A shows a graph showing spherical aberration of the fourthembodiment of the objective lens, FIG. 27B is a graph showing astigmaticaberration thereof and FIG. 27C is a graph showing distortion aberrationthereof.

[0052]FIG. 28 shows a lens structure of a fifth embodiment of anobjective lens for an optical pickup according to the present invention.

[0053]FIG. 29A shows a graph showing spherical aberration of the fifthembodiment of the objective lens, FIG. 29B is a graph showing astigmaticaberration thereof and FIG. 29C is a graph showing distortion aberrationthereof.

BEST MODE FOR CARRYING OUT THE INVENTION

[0054] Referring to the drawings, an objective lens used in an opticalpickup device, an optical pickup device employing this objective lensand the optical disc device, according to the present invention, arehereinafter explained in detail.

[0055] In the following embodiments, the present invention is applied toan optical disc device for recording and/or reproducing the informationon or from a disc-shaped recording medium having a track pitch on theorder of 0.06 μm which is narrower than the track pitch of the recordingmedium provided in a CD (Compact Disc) as a disc-shaped recordingmedium, for example, a DVD (Digital Video Disc/Digital Versatile Disc),that is a disc-shaped recording medium having the track pitch of therecording track narrowed to elevate the information recording density.

[0056] First, an optical disc device, employing an optical pickup,employing in turn an objective lens according to the present invention,is explained.

[0057] The optical disc device 1 according to the present invention issuch a disc device in which recording signals recorded with an elevatedrpm can be read and written at an elevated speed, and employs an opticalrecording medium having the track pitch narrowed significantly toelevate the recording density, such as DVD (Digital Video/VersatileDisc). The optical disc device 1 according to the present invention isused as an external storage device for an information processing device,such as a personal computer.

[0058] Referring to FIGS. 3 and 4, the disc driving device 1 includes amechanical frame 2, having various mechanical units arranged thereon.The upper, left, right, forward and rear portions of the mechanicalframe 2 are covered by a cover member 3 and a front panel 4, which aresecured to the mechanical frame 2 by suitable fastening means, such asset screws.

[0059] The cover member 3 is made up integrally by a an upper plateportion 3 a, lateral surface portions 3 b, 3 b, depending from both sideedges of the upper plate portion 3 a, and a rear plate portion, notshown. The front panel 4 has a transverse elongated opening 4 a. A door5 for opening/closing the opening 4 a is rotatably supported by thefront panel 4 with the upper end of the door as a fulcrum. The frontpanel 4 is provided with plural operating buttons 6 for performingvarious operations.

[0060] The mechanical frame 2 includes a mechanical unit mountingsurface portion 7 a, and lateral portions 7 b, 7 b, upstanding from bothlateral sides of the mechanical unit mounting surface portion 7 a. Onthe front side end of the mechanical unit mounting surface portion 7 ais arranged a loading unit 8 including, for example, cam plates orgears.

[0061] A disc tray 9 is carried on the mechanical frame 2 for movementin the fore-and-aft direction, that is in the direction indicated byarrows A1 and A2 in FIG. 4. The disc tray 9 includes a transverselyelongated insertion opening 9 a and a disc setting recess 9 b in whichto set a disc-shaped recording medium 100, which is sometimes referredto below simply as an optical disc. When the optical disc 100 is set onthe disc setting recess 9 b, the disc tray 9 is moved by the loadingunit 8 so as to be protruded through the opening 4 a of the front panel4 to outside the main body unit of the device. When the information isrecorded or reproduced from the optical disc 100, the optical disc 100is intruded into the inside of the main body unit of the device as theoptical disc 100 is set on the disc setting recess 9 b.

[0062] On the mechanical unit mounting surface portion 7 a, a movableframe 10 is supported for rotation, with the rear end thereof as arotational fulcrum point, as shown in FIG. 4.

[0063] The movable frame 10 is provided with a motor unit 11 for causingrotation of the optical disc 100. The motor unit 11 includes a disctable 11 a and a driving motor 11 b. On the movable frame 10 is carriedan optical pickup 12 so that the optical pickup is movable by a guideshaft and a lead screw, not shown, along a radial direction of theoptical disc 100 loaded on the disc table 11 a.

[0064] On the movable frame 10 is mounted a feed motor 13 adapted forrotating the lead screw. Consequently, when the lead screw is rotated bythe feed motor 13, the optical pickup 12 is moved in the directioncorresponding to the direction of rotation as the optical pickup isguided by the guide shaft.

[0065] In the disc driving device 1 of the present invention, when theoptical disc 100 set on the disc setting recess 9 b of the disc tray 9is intruded into the inside of the device, held by suitable means on thedisc table 11 a and is run in rotation along with the disc table 11 a bythe operation of the driving motor 11 b of the motor unit 11, theinformation is recorded on or reproduced from the optical disc 100, asthe optical pickup 12 is moved along the radius of the optical disc 100.

[0066] The structure of the optical pickup 12, adapted for recording orreproducing the information on or from the optical disc 100, is nowexplained in detail.

[0067] The optical pickup 12 is shown in FIG. 4, includes opticalelements, such as a light emitting element, emitting laser light, or alight receiving element, and a biaxial actuator, not shown, forsupporting an objective lens 15, loaded on a movable base 14 supportedby the movable frame 10 by the guide shaft and the lead screw, notshown. The optical pickup 12 includes an objective lens 15, carried bythe biaxial actuator, not shown, a laser light emitting element 16 forradiating the laser light with the wavelength not larger than 420 nm,and a collimator lens 17 for collimating the laser light radiated fromthe laser light emitting element 16, as schematically shown in FIG. 5.The laser light, radiated from the laser light emitting element 16,comprised of the semiconductor laser, is collimated by the collimatorlens 17 so as to be converged by the objective lens 15 on the recordingsurface of the optical disc 100.

[0068] The optical pickup 12 is used for recording the information onthe optical disc 100, having a high information recording density, forreproducing the information recorded on the optical disc 100. The laserlight emitting element 16, used in this optical pickup 100, generatesthe laser light with a wavelength not larger than 420 nm, which isshorter than the wavelength of 780 nm generated by a laser lightemitting element of the conventional CD standard, specifically, thelaser light with a wavelength on the order of 400 to 410 nm. The highfrequency current is superposed on the driving current to reduce thelaser noise so that the laser light wavelength will be varied in shortperiods.

[0069] In recording the information on the optical disc 100, the laserlight of high energy is radiated from the laser light emitting element16 and collimated by the collimator lens 17. The laser light thuscollimated falls on the objective lens 15 so as to be converged on therecording layer of the optical disc 100 to form a laser spot thereon.The energy of this laser light causes the recording layer to undergoe.g., phase change to form pits corresponding to the recordedinformation.

[0070] In reproducing the information recorded on the optical disc 100,the laser light of an energy lower than in recording the information isradiated from the laser light emitting element 16 and collimated by thecollimator lens 17. The laser light thus collimated falls on theobjective lens 15 so as to be converged on the recording layer of theoptical disc 100 to form a laser spot thereon. The laser light reflectedby the recording layer of the optical disc 100 is transmitted through anoptical path which is the reverse of the optical path of the laser lightincident on the optical disc 100 so as to be detected by a lightreceiving system, not shown, including the light receiving element inthe optical pickup 12.

[0071] The operating length of the objective lens 15, that is thedistance from the surface of the lens of the lens set of the objectivelens 15 closest to the optical disc 100 to an image point, that is therecording layer on which is converged the laser light of the opticaldisc 100, is set to not less than 0.5 mm.

[0072] The objective lens 15 of the present invention is now explainedin more detail.

[0073] As shown in FIG. 5, and as also shown in FIGS. 20, 22, 24, 26 and28 showing concrete embodiments, as later explained, the objective lensaccording to the present invention includes a first lens set GR1comprised of a first lens L1 and a second lens set GR2 comprised of asecond lens L2. The first lens L1 is a hybrid type lens having arefractive and diffractive compound surface which is composed of arefractive surface S_(2r) as a base surface, and a diffractive surfaceS_(2d) added thereto, while the second lens L2 is a single lens of themeniscus shape both surfaces of which are comprised of asphericalsurfaces, with the second lens having a positive refractive power. Therefractive and diffractive compound surface is referred to below simplyas the compound surface.

[0074] The first lens set GR1 plays the role of correcting chromaticaberration, while the second lens set GR2 plays the role of convergingthe beam spot of the laser light to a preset size.

[0075] In the following explanation, the surface numbers of the lensesand other constituent elements are counted sequentially in the order of1, 2, 3, . . . from the light source (laser emitting element). It isnoted that [S_(i)], [r_(i)] and [d_(i)] denote the number i surface ascounted from the light source side, the radius of curvature of thenumber i surface S_(i) as counted from the light source and the spacingon the optical axis between the number i surface and the number i+1surface as counted from the light source, respectively. The asphericalsurface is defined by the following equation 1: $\begin{matrix}{x = {\frac{c\quad h^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}h^{2}}}} + {A\quad h^{4}} + {B\quad h^{6}} + {C\quad h^{8}} + {D\quad h^{10}} + {E\quad h^{12}} + {F\quad h^{14}} + {G\quad h^{16}} + {H\quad h^{18}} + {J\quad h^{12}}}} & (1)\end{matrix}$

[0076] where [x] is the distance from the tangential plane of an apexpoint of an aspherical surface of a point on the aspherical surfacehaving a height h from the optical axis, [c] is a curvature of the apexpoint of the aspherical surface (=1/R), [k] is a cone constant, [A],[B], [C], [D]. [E]. [F], [G], [H] and [J] denote the fourth to twentiethorder aspherical coefficients, respectively.

[0077] In general, the diffractive optical element, used as adiffraction surface, is classed into an amplitude type and a phase type.The diffractive optical element, used as a diffraction surface S_(2d) ofthe objective lens 15, is of the phase type, in particular a bladedhologram in the form of blades, from the perspective of efficiency. Inthis bladed hologram, similarly to the routine hologram, the polarcoordinates on a substrate are specified, using a polynominal, as shiftcoefficients of the aspherical phase on each plane when two point lightsources are assumed to be at points of infinity at the time ofmanufacture. The coefficients of the above polynominal give the opticalpath difference (OPD) at the reference wavelength of diffraction in mm.That is, the optical path difference by diffraction at a point of aheight R from the optical axis on the diffraction surface is defined by

OPD=CIR ² +C2R ⁴ +C3R ⁶ +C4R ⁸ +C5R ¹⁰ +C6R ¹² +C7R ¹⁴ +C8R ¹⁶ +C ₉ R ¹⁸+CI0R ²⁰.

[0078] The actual shape of the diffraction surface is interruptedlychanged in order to produce diffraction. That is, since the optical pathdifference between the optical path in a medium with a refractive indexN and the optical path in air is given by t(N−1), the step difference dof the respective ring zones (elements) of the diffraction surface isgiven by

d=λ/(N−1)

[0079] or integer number multiples thereof. In the above equation, λ isthe design wavelength in nm. The blade shape of the diffraction surfacegives to the surface shape the depth yielded as the remainder obtainedon dividing the optical difference OPD by the wavelength λ.

[0080] The first lens L1 is formed of glass or resin, and includes acompound surface S₂ obtained on adding a diffractive surface S_(2d) to arefractive surface S_(2r), that is a reference surface which defines thediffractive surface. The diffractive surface S_(2d) is formed directlyon the refractive surface S_(2r), using a metal mold manufactured bymachining using a diamond byte. Alternatively, the diffractive surfaceS_(2d) carrying a phase transmission type hologram may be formed on thesurface of a resin layer 18 deposited by any suitable method on therefractive surface S_(2r). By employing the compound surface S₂, havingthe diffraction surface S_(2d), it is possible to increase the numericalaperture (NA) without increasing the lens diameter. The diffractionsurface S_(2d) is formed to have a fine staircase shape when seen incross-sectional view, as explained subsequently. The refractive surfaceS_(2r) of the objective lens 15, as the base surface, is a parabolicsurface, with a cone constant k=−1.

[0081] With the objective lens 15, a third surface S₃ and a fourthsurface S₄ of the second lens L2 are formed as aspherical surfaces.

[0082] The objective lens 15, made up by two lens groups, is arrangedwithin a lens barrel 19 formed of a suitable material, such as syntheticresin, as shown in FIG. 5. Although not shown in detail, the lens barrel19 is of a substantially cylindrica shape, having its both ends opened.The first and second lenses L1, L2 are introduced into the inside of thelens barrel 19 from the end openings and secured in position such as bycentering. By arranging the objective lens 15, comprised of two lenssets, in the lens barrel 19 subject to advance optical adjustment of thecomponent lenses, such as centering, it is possible to improveoperational performance at the time of assembling the objective lens 15into the optical pickup 12.

[0083] Since the first lens set GR1 is made up by the planar firstsurface S1 and the compound surface S2 formed by a refractive surfaceS_(2r) having a negative refractive power and a refractive surfaceS_(2d) having a positive refractive power, the total quantity of therefractive power is equal to zero (0). By setting the refractive powerof the first lens set GR1 to zero, the first lens set GR1 operates as acompletely plan-parallel plate with respect to the light of the designwavelength, or operates simply as a lens of an extremely smallrefractive power, even if the wavelength undergoes shifting, so that,when the first lens set GR1 is inserted into a space between thecollimator lens 17 and the second lens set GR2 operating for convergingthe laser light, the offset, tilt or the spacing can be designed with abroader tolerance. By employing the first lens set GR1 solely forcorrecting the chromatic aberration, with the operation of forming abeam spot being taken charge of only by the second lens set GR2, therole sharing between the lens sets can be more well-defined to providefor facilitated designing of the lens sets.

[0084] The objective lens 15 has a working distance, that is a distancefrom the trailing lens surface (fourth surface S₄) to the image point,equal to not less than 0.5 mm. With the lens comprised of two lens sets,with a large numerical aperture, such as a solid immersion lens (SIL),the working distance is sometimes on the order of 0.1 mm. If the workingdistance is this short, there is raised a problem that the collisionbetween the objective lens and the optical disc is inevitable. For thisreason, the present invention provides for the working distance betweenthe trailing end surface and the image point which is to be 0.5 mm ormore. On the other hand, the objective lens 15 is designed to have aneffective focal length not larger than 1.875 mm.

[0085] The achromatic condition in converging the laser light to thethreshold of diffraction in the objective lens 15, that is the conditionfor correction of the chromatic aberration, is now explained.

[0086] In general, the achromatic condition for a lens combined from arefractive type lens and a diffractive type lens, for a light source oflight the wavelength of which is changed in a range from ±δ (nm) withrespect to the wavelength λ, is derived as follows:

[0087] That is, the Abbe number v_(r) for a range of the wavelength λ±δ,referred to below as partial Abbe number, may be expressed by thefollowing equation 2: $\begin{matrix}{v_{d} = \frac{N - 1}{N_{+ \delta} - N_{- \delta}}} & (2)\end{matrix}$

[0088] where N, N_(+δ) and N_(−δ) denote the refractive indices of thevitreous material for the wavelengths of λ, λ_(+δ) and λ_(−δ),respectively.

[0089] On the other hand, the partial Abbe number v_(d) of thediffractive type lens may be expressed by the following equation 3:$\begin{matrix}{v_{d} = {\frac{\lambda}{\left( {\lambda + \delta} \right) - \left( {\lambda - \delta} \right)}.}} & (3)\end{matrix}$

[0090] With the focal lengths f_(r), f_(d) of the refractive type lensand the diffractive type, lens respectively, the synthetic lens obtainedon synthesizing these lenses has a focal length f which is given by thefollowing equation 4: $\begin{matrix}{\frac{1}{f} = {\frac{1}{f_{r}} + \frac{1}{f_{d}}}} & (4)\end{matrix}$

[0091] while the achromatic condition of the image point on the opticalaxis is expressed by the equation 5:

f _(r) ·v _(r) +f _(d) ·v _(d)=0  (5).

[0092] From the above equations 4 and 5, the focal length f_(r) of therefractive lens and the focal length f_(d) of the diffractive type lens,shown by the following equation 6: $\begin{matrix}\begin{matrix}{f_{r} = {f \cdot \frac{1 - v_{d}}{v_{r}}}} \\{f_{d} = {f \cdot \frac{1 - v_{r}}{v_{d}}}}\end{matrix} & (6)\end{matrix}$

[0093] are derived.

[0094] Meanwhile, the partial Abb number v_(r) of the refractive lens isdetermined by the refractive index of the lens material, whilst thepartial Abb number V_(d) of the diffractive lens is determined by thewavelength of the laser light used. If the fact that the refractiveindex of the lens material is varied with the wavelength, it may be saidthat the partial Abb number v_(r) of the refractive lens is determinedby the lens material and the wavelength of the laser light being used,while the partial Abb number v_(d) of the refractive lens is determinedsolely by the wavelength of the laser light being used.

[0095] It should be noted that the refraction and the on-axis achromaticcondition in the refraction-diffraction compound objective lens 15 ofthe present invention may be uniquely determined by the wavelength λ ofthe laser light used, fluctuation of the laser light, that is variationsin the wavelength δ, lens material type, beam diameter of the incidentlaser light and the numerical aperture (NA).

[0096] With the objective lens for the optical pickup according to thepresent invention, the design wavelength and the beam diameter of thelaser light and the numerical aperture (NA) of the objective lensrepresent fixed parameters, so that, if once the lens material type isdetermined, the on-axis achromatic condition in therefraction-diffraction compound objective lens is determined. Forexample, if NA=0.8, the beam diameter of the laser light is 3 mm, λ=410nm, δ=±10 nm and the lens material is LAH53 (trade name of a productmanufactured by KK Ohara), the focal length of the refractive lens f_(r)is 2.18 mm, while that of the diffractive lens f_(d) is 13.31 mm.

[0097] In the objective lens 15 of the present invention, the first lensset GR1 and the second lens set GR2 play the role of correcting thechromatic aberration and of converging the laser beam spot to a presetsize, respectively. The refractive index of the first lens set GR1 iszero, such that the sum total of the second lens set GR2 proves therefractive power of the objective lens 15. This may be expressed by thefollowing equations 7 and 8: $\begin{matrix}{{\frac{1}{f_{1}} + \frac{1}{f_{d}}} = 0} & (7) \\{f_{2} = {f.}} & (8)\end{matrix}$

[0098] With the objective lens 15 of the present invention, thesynthesized focal length of the refractive lens of the first lens setGR1 (having the first surface S₁ and the second surface S₂) and thesecond lens L2 of the second lens set GR2 proves the focal length fr ofthe totality of the refractive lenses, so that the following equation 9:$\begin{matrix}{\frac{1}{f_{r}} = {\frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{d}{f_{1} \cdot f_{2}}}} & (9)\end{matrix}$

[0099] is obtained.

[0100] If, in designing an objective lens for an optical pickup, thefocal length and the lens material are determined, f, f_(r) and f_(d)are determined automatically, and hence the focal length f₁ of therefractive type lens of the first lens set and the distance d betweenthe first and second lens sets are also determined unequivocally.

[0101] Meanwhile, the position of the principal point of the refractivetype lens is determined by the refractive index of the lens material,radius of curvature of each lens surface and the spacing between thesurfaces, and is not fixed at a constant position. Thus, the distancebetween the principal points of the diffractive type lens and thesynthesized refractive type lens is changed with design conditions.Therefore, the paraxial solution has to be derived by carrying outbending at the time of designing so that the above equation 6 will bemet.

[0102] The designing of the staircase shape of the diffractive surfaceS_(2d) of the compound surface S₂, which forms the phase diffractiontype optical component, is hereinafter explained.

[0103] The sag ASP(r) of the diffractive surface S_(2d) of the compoundsurface S₂ is defined, by a software for optical designing “CODEV”, asindicated by the following equation 10: $\begin{matrix}{{{ASP}(r)} = {\frac{c\quad r^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}r^{2}}}} + {A\quad r^{4}} + {B\quad r^{6}} + {C\quad r^{8}} + {D\quad {r^{10}.}}}} & (10)\end{matrix}$

[0104] If, in the above equation 10, k=−1, the second order coefficientis c/2, so that the above equation reduces to a polynominal with onlyeven number orders shown by the following equation 11: $\begin{matrix}{{{ASP}(r)} = {{\frac{c}{2}r^{2}} + {A\quad r^{4}} + {B\quad r^{6}} + {C\quad r^{8}} + {D\quad {r^{10}.}}}} & (11)\end{matrix}$

[0105] On the other hand, the phase distribution Φ(r) of the diffractivesurface S_(2d) of the compound surface S₂ is defined by a polynominalcomposed only of the even number orders, as indicated by the followingequation 12:

Φ(r)=C ₁ r ² +C ₂ r ⁴ +C ₃ r ⁶ +C ₄ r ⁸ +C ₅ r ¹⁰  (12)

[0106] Thus, up to optical designing, the phase is treated as acontinuously changing function, as indicated in the equation 12.

[0107] In the surface designing of the diffractive surface, the phaseperiodicity is exploited so that the remnant phase obtained onsubtracting an integer multiple period from the phase period is used asa new phase distribution. Thus, the shape of the diffractive surfaceproves a discrete blade shape. Since this blade type discrete phase iscalculated as the actual thickness of the bulk, the shape of thediffractive surface material is a discrete blade shape, with the heightbeing a thickness affording a phase difference equal to an integermultiple of the wavelength.

[0108]FIG. 6 illustrates the cross-sectional shape of the asphericalsurface and that of the diffractive surface. The cross-sectional shapeof the aspherical surface ASP_Sag is given by the above equation 11,while the cross-sectional shape of the diffractive surface DOE_Sag ofthe diffractive surface is given by the above equation 12. In order forthe phase of the diffractive surface to be of the same dimension as thatof the sag of the refractive surface, the phase is divided by arefractive index difference.

[0109]FIG. 7 illustrates the shape of the refraction-diffractioncompound surface. The sag of the refraction-diffraction compound surfaceSag(r) may be expressed as indicated in the following equation 13:$\begin{matrix}{{{Sag}(r)} = {{{ASP}(r)} + {\frac{\Phi (r)}{N - 1}.}}} & (13)\end{matrix}$

[0110] If, in the above equation 13, Sag(r)=0, the phase is a completesurface, so that the incident light can be radiated unaffected. In thiscase, the diffractive surface is of a vertical staircase shape with thethickness giving a phase equal to an integer multiple of the wavelengthas one step. That is, it suffices to select the solution of Sag(r)=0 sothat coefficients of the respective order numbers will cancel oneanother to give zero, as indicated by the following equation 14:$\begin{matrix}{{k = {- 1}}{C_{1} = {\left( {N - 1} \right)\frac{C}{2}}}{C_{2} = {\left( {N - 1} \right)A}}{C_{3} = {\left( {N - 1} \right)B}}{C_{4} = {\left( {N - 1} \right)C}}{C_{5} = {\left( {N - 1} \right){D.}}}} & (14)\end{matrix}$

[0111] The coefficient of the second order indicates that the refractivepowers of the refractive surface and the diffractive surface in theparaxial area cancel one another to give zero.

[0112] Referring to the drawings, the effect of the staircase shape ofthe diffractive surface as described above is hereinafter explained.

[0113]FIGS. 8 and 9 illustrate machining problems and countermeasures tobe taken in case the width of a distal end 20 a of a byte 20 formachining a metal mold used for transcribing the diffractive surfacedirectly on the diffractive surface S_(2r) of the first lens L1 or onthe surface of the layered resin layer 18, in the corse of manufactureof the objective lens 15, is of an non-negligible size with respect tothe widths of respective ring zones 22 a, 22 a, . . . of the blade shape(shape of transcription) formed on a transcription surface 21 of a metalmold.

[0114] That is, when the diffractive surface is of a blade shape, thetranscription surface 21 of a metal mold to be machined is fed in thedirection indicated by arrow X₁, while the byte 20 is fed in thedirection indicated by arrow Y₁, whereby the transcription surface 21 ismachined to form the blade shape or transcription surface 22. It shouldbe noted that the shape of the distal end 20 a of the byte 20, if acuteon visual check, is not acute as compared to the size of the ring zones22 a, 22 a, . . . of the fine blade shape. Moreover, the shape of thedistal end 20 a cannot be made acute as compared to the size of theblade-like ring zones 22 a, 22 a, . . . . Consequently, non-machinedportions 25, 25, . . . which are unable to be machined by the distal end20 a of the byte 20, are left between the ring zones 22 a, 22 a, . . .of the fine blade shape 22 of the transcription surface 21 of the metalmold.

[0115] Moreover, if the shape of the diffractive surface is thestaircase shape as shown in FIG. 9, the transcription surface 21 of themetal mold to be machined is fed in the direction indicated by arrow X₁,while the byte 20 is fed in the direction indicated by arrow Y₁, as inthe case of the blade shape of the diffractive surface, whereby thetranscription surface 21 is machined to form the staircase shape 26.However, if the diffractive surface is of the staircase shape, thefacets making up the ring zones 26 a, 26 a, . . . of the staircase shapeinclude no facets disposed obliquely with respect to the surface makingup the distal end 20 a of the byte 20, so that there is no fear of thenon-machinable portions being left, such as those described above.

[0116] The graph of FIG. 10 shows the relation between the width b ofthe non-machinable portions 27 a, 27 a, . . . produced on transcriptionof the non-machinable portions 25, 25, . . . of the transcriptionsurface 21 in the blade type diffraction grating 27, having a constantperiod L (blade width: 4 μm), as shown in FIG. 11, and the diffractionefficiency. FIG. 12 shows the relation between the depth h of theundulations 29 and the transmittance in an undulating irregularitiessurface structure 28, which is the shape of periodic finemicro-irregularities, with a period of d, produced on the transcriptionsurface (machined surface) 21 as movement trajectory of the byte 20, asshown in FIG. 13.

[0117] Meanwhile, the diffractive surface which gives the 100%diffraction efficiency of the difference type lens has the blade-likecross-sectional shape. The blade height (sag) is a thickness which givesan integer multiple of the wavelength. If a diffraction type lens isproduced on a surface, the cross-sectional shape is a blade shape.

[0118] If, in designing the diffraction type lens, the coefficients ofthe polynominal of the equation 13 are selected for cancelling the sag,the cross-sectional shape of the diffraction surface is the staircaseshape transcribed by the transcription surface 21 of the metal mold, asshown in FIG. 9. The height of the staircase (sag) proves a thicknesswhich gives the phase difference equal to an integer multiple of thewavelength, as in the case of the surface described above. It should benoted that, since the staircase shape may be thought of as a specialcase of the blade shape, the theoretical diffraction efficiency of thestaircase shape is the same as that for the blade shape.

[0119] However, when the blade shape is afforded to the transcriptionsurface 21 of the metal mold by machining with the byte 20, and thediffraction surface is of the blade shape in the cross-section includingthe inclined surface, the non-machinable portions 25, 25, . . . , thatremain non-machined, are produced due to the shape of the distal end 20a of the byte 20, as shown in FIG. 8. If these non-machinable portions25, 25, . . . become non-negligibly increased in size as compared to theblade shape 22, the diffraction efficiency is lowered in proportion tothe size of the blade shape 22, as shown in FIG. 10. The non-machinableportions 25, 25, . . . become smaller the smaller (the acuter) the shapeof the distal end of the byte 20.

[0120] In machining the transcription surface 21 of the metal mold,employing the byte 20, there is produced an undulated pattern 29 of afine irregular shape, which is the movement trajectory of the byte 20,on the inclined surface of the blade shape, as shown in FIG. 13. If theperiod d of the undulating irregularities surface structure 28 as therepetitive shape of the undulated pattern 29 and the depth h of theundulated pattern 29 is of the same order of magnitude as or larger thanthe design wavelength of the lens, the transmittance is lowered inproportion to the value of the relative size of the period d to thedepth h. In the case of FIG. 12, the period d is 0.8 μm. This undulatedpattern 29 becomes smaller the smaller (acuter) the shape of the distalend 20 a of the byte 20.

[0121] If the blade shape 22 is formed on the transcription surface 21by machining employing the byte 20, there is produced a certain rulebetween the diffraction efficiency of the diffraction surface of thelens produced on transcription, and transmittance, depending on theshape of the distal end 20 a of the byte 20.

[0122] If conversely the staircase shape 26 shown in FIG. 9 is to beformed on a transcription surface 21 of the metal mold, by machiningemploying the byte 20, no non-machinable portions, such as thenon-machinable portions 25, 25, . . . , produced in the case of theblade shape 22, are left, because no inclined facet is included in thefacet produced on machining, with the consequence that the producedcross-sectional shape is close to the ideal cross-sectional shape. Thus,if the diffraction surface is that obtained on transcription of thestaircase shape 26, it becomes possible to eliminate the factor oflowering the diffraction efficiency. Moreover, since there is noconstraint on the shape of the distal end 20 a of the byte 20, such abyte shaped to help suppress the formation of the undulatingirregularities surface structure 28 shown in FIG. 13 may be selected asthe byte 20 without dependency on the distal end shape, thus decreasingthe factor of lowering the transmittance.

[0123] The influence of a fine irregular shape 39 formed on the compoundsurface of the objective lens 15 by transcription of the surfaceundulated structure 28 as the repetition of the saw tooth likeprojections 29 of the fine irregular shape formed on the transcriptionsurface of the metal mold used for molding the objective lens 15 is nowexplained in detail.

[0124] The transmission and reflection of the laser light on thecompound surface S₂ of the objective lens 15, carrying the fineirregular shape 39 as a result of transcription of the surface undulatedstructure 28 of the metal mold, are shown in FIG. 14. That is, part of alaser light beam 40 incident on the objective lens 15 proves a reflectedlight beam from the surface of the fine irregular shape 39 to lower thetransmittance. A transmitted laser light beam 41, incident on andtransmitted through the objective lens 15, is refracted in unforseendirections, depending on the surface shape of the fine irregular shape39, thus generating stray light, such as stray light 41 a, thus furtherworsening the transmittance of the surface carrying the fine irregularshape 39.

[0125] It should be noted that the, with the compound surface S₂ of theobjective lens 15, the period of the surface undulated structure 28 ofthe metal mold and hence the period and the amplitude of the fineirregular shape 39 formed on the compound surface S₂ of the objectivelens 15, can be controlled by optimally controlling the movement of themachining byte with respect to the material constituting the metal moldduring machining the transcription surface 21 of the mold for producingthe objective lens 15. The transmittance of the laser light of a presetwavelength can be controlled by controlling the period and the amplitudeof the fine irregular shape 39 formed on the compound surface S₂ topreset conditions.

[0126] The fact that the transmittance of the objective lens 15,carrying the fine irregular shape 39, is further explained in detail.

[0127] For controlling the transmittance of the laser light of a presetwavelength, the surface undulated structure 28, formed on thetranscription surface 21 of the metal mold for molding the objectivelens 15, is formed to a concentric shape having a periodic structurewith a period d on the order of about one half the reference wavelength(λ) of the laser light and with an amplitude h on the order of onequarter the reference wavelength (λ) of the laser light. Bytranscription of this surface undulated structure 28, the fine irregularshape 39 formed on the compound surface S₂ of the objective lens 15 hasthe similar period d and the amplitude h.

[0128] Preferably, the orientation of the fine irregular shape 39 iscoincident with that of the transcribed blade shape, that is, the fineirregular shape is similar to the transcribed blade shape, with theblade shape being in register with the inclined surface of the fineirregular shape.

[0129] The process of forming the fine irregular shape 39 with theperiod d and the amplitude h as described above is now explained withreference to FIGS. 15 and 16.

[0130]FIG. 15 shows the relation between the amplitude h and thetransmittance in routine red laser light Re with the wavelength of 650nm, and with blue laser light Bu, with the wavelength of 405 nm, used inthe present invention, for the period d of the fine irregular shape 39as the transcribed shape of the undulated structure equal to 0.5 μm.

[0131]FIG. 16 shows the relation between the amplitude h and thetransmittance in routine red laser light Re with the wavelength of 650nm, and with blue laser light Bu with the wavelength of 405 nm, for theperiod d of the fine irregular shape 39 as the transcribed shape of theundulated structure equal to 0.2 μm.

[0132] It is seen from the graph of FIG. 15 that, with the fineirregular shape 39, having the period d of 0.5 μm, a transmittance onthe order of 90% is possible when the amplitude h of the fine irregularshape 39 is 0.33 μm, as an amplitude which gives the phase difference onthe order of one quarter of the wavelength of the red laser light Rewith red laser light Re. However, with the laser light Bu, only atransmittance on the order of 70% can be achieved when the amplitude dof the fine irregular shape 39 is 0.2 μm, as an amplitude which affordsa phase difference on the order of one quarter of the wavelength of theblue laser light.

[0133] It may also be seen from the graph of FIG. 16 that, with the fineirregular shape 39 having a period d of 0.2 μm, a transmittance on theorder of 90% is achieved even if the amplitude h of the fine irregularshape 39 for the red laser light Re is set to 0.33 μm, as an amplitudewhich gives the phase difference on the order of one quarter of thewavelength of the red laser light Re, or if the amplitude d of the fineirregular shape 39 for the blue laser light Bu is set to 0.2 μm, as anamplitude which gives the phase difference on the order of one quarterof the wavelength of the blue laser light Bu.

[0134] With the objective lens 15 of the present invention, in which thediffraction surface S_(2d) of the compound surface S₂ is of a finestaircase-like cross-sectional shape and in which the period d and theamplitude h of the fine irregular shape 39 formed on the staircase-likesurface, are set as described above, it is possible to alleviate theproblem of the lowering of the diffraction efficiency and that of thelowering of the transmittance simultaneously.

[0135] By referring to the drawings, another embodiment of an objectivelens according to the present invention is now explained.

[0136] Similarly to the objective lens 15, described above, an objectivelens 55, shown in FIGS. 17 and 18, is of a double lens set structurecomprised of a first lens set GR1, made up by a first lens L1 of thehybrid type lens having a first surface S₁ as a refractive surface and asecond surface S₂, combined together with the first lens surface to forma refraction-diffraction compound surface, and a second lens set GR2,made up by a second lens L2, as a single lens having a third surface S3and a fourth surface S₄, both being aspherical surfaces.

[0137] The portions common to those of the above-described objectivelens 15 are depicted by common reference numerals and detaileddescription therefor is omitted for simplicity.

[0138] The double set structure objective lens 55 is arranged in a lensbarrel 58, formed of any suitable material, such as synthetic resin, asshown in FIG. 18. The lens barrel 58 is formed to a substantiallycylindrical shape, having both ends opened, with the first lens L1 andthe second lens L2 being introduced into the inside thereof via both endopenings and secured in position as the lenses are centered or adjustedin the respective mounting positions, as shown in FIG. 18. If the doubleset structure objective lens 55 is optically adjusted in its position,such as centering the respective constituent lenses in advance, andarranged in the lens barrel 58, it is possible to improve the workingperformance in assembling the objective lens 55 in an optical pickup 12,as shown in FIG. 17.

[0139] The objective lens 55 is provided with an aperture 59 between thefirst lens set L1 and the second lens set L2. Specifically, the aperture59 is provided in the vicinity of the third surface S₃ towards the lightsource of the second lens set GR2, as shown in FIG. 18, in order tolimit the light volume of the laser light incident on the second lensset GR2. The aperture is formed of the same material as the lens barrel58 or formed to a substantially ring shape as a separate member and issuitably mounted by suitable means, such as by adhesion, to an innerperipheral wall 58 a of the lens barrel 58. By arranging the aperture 59in the vicinity of the third surface S₃ towards the light source of thesecond lens set GR2 having a large refractive power (second lens L2),only the portion of the lens having a satisfactory optical performanceis restrictively used without employing the lens rim to stabilize theperformance to provide for facilitated correction of various aberrationtypes.

[0140] With the present objective lens 55, the first lens set GR1 iscomprised of a first planar surface S₁ and a second compound surface S₂made up by a refractive surface S_(2r) having a negative refractivepower and a diffractive surface S_(2d) having a positive refractivepower, whereby the sum total of the refractive power is equal to zero(0).

[0141] With the present objective lens 55, the working distance, that isthe distance between the trailing lens surface (fourth surface S₄) andthe image point, is set to not less than 0.5 mm, with the effectivefocal length being not larger than 1.875 mm.

[0142] Referring to the drawings, a further embodiment of an objectivelens 65 of the present invention is hereinafter explained.

[0143] Referring to FIG. 19, the objective lens 65 has a basic lensstructure similar that of the objective lens 55 shown in FIG. 18, and ismade up by a first lens set GR1 as a first lens L1 of the hybrid typelens having a second surface S₂ which is combined from a refractivesurface S₂, and a diffractive surface S_(2d), and a second lens set GR2,having a positive refractive power and which is made up by a second lensL2, as a single lens having a third surface S₃ and a fourth surface S₄,both being aspherical surfaces.

[0144] The first lens L1, forming the objective lens 65, is formed of atransparent resin material. The compound surface (second surface) S₂ iscomprised of a refractive surface S_(2r) of the first lens L1 ofsynthetic resin as a base surface (reference surface for defining thediffractive surface) and a diffractive surface S_(2d) provided to therefractive surface S_(2r) comprised of a phase transmission typehologram of a blade structure formed by transcribing to the refractivesurface S₂, the blade shape formed by a suitable method, such asmachining a transcription metal mold by a diamond byte.

[0145] The objective lens 65 is unitarily formed by the first lens L1and a lens barrel 60 of the same material, as shown in FIG. 19. Thus,the lens barrel 60 has its one end closed by the first lens L1, whilehaving its other end opened. With the present objective lens 65,unitarily formed by molding from the first lens L1 and the lens barrel60, it is unnecessary to perform optical adjustment, such as centeringor positioning of the first lens L1.

[0146] The second lens L2 is introduced into the inside of the lensbarrel 60, formed as one with the first lens L1, and is secured inposition as it is centered and adjusted in its mounting position. Thatis, the optical adjustment in the lens barrel 60 is required only as tothe second lens L2. It is therefore only sufficient to perform theoptical adjustment with respect to the second lens L2, so that theoperational performance in assembling the objective lens 65 in theoptical pickup 12 may be improved appreciably.

[0147] Since the objective lens 65 includes the first lens L1 and thelens barrel 60 formed unitarily of synthetic resin, as described above,the objective lens in its entirety may be reduced in weight.

[0148] The objective lens 65 is also provided with an aperture 61, inthe form of a belt-shaped thin film, by vacuum-depositing suitable metalon the entire outer periphery of the third surface S₃ of the second lensL2 facing the light source side. The reason the aperture 61 is formed asone with the third surface S₃ of the second lens L2 is that, since thefirst lens L1 and the lens barrel 60 are formed as one integral unit, itis impossible to form the aperture integrally within the inside of thelens barrel 60 from the perspective of rapping at the time of molding.

[0149] By providing the aperture 61 in this manner on the third surfaceS₃ of the second lens L2 of the objective lens 65 facing the lightsource side, it is possible to reduce the assembling error of theaperture 61 to the second lens L2 of the objective lens 65, theassembling error of the aperture 61 to the second lens L2 may be reducedto zero, while only the lens portions exhibiting satisfactory opticalperformance is restrictively used to provide for stabilized performance.

[0150] With the present objective lens 65, it is again possible tocorrect the chromatic aberration at the image point on the optical axiswith respect to the light of a wavelength within several nm about 420 nmor less as a reference.

[0151] An embodiment of an objective lens 15 according to the presentinvention is now explained.

[0152]FIG. 20 shows a lens structure of the first embodiment of theobjective lens 15 according to the present invention. This objectivelens 15 is made up by a first lens set GR1, formed by a first lens L1comprised of a plano-concave lens of glass, made up of a refractivesurface S_(2r) and a composite surface S₂ comprised of a diffractivesurface, layered on the refractive surface S_(2r) as base surface, and asecond lens set GR2, formed by a second lens 12 as a single asphericalglass molded lens with a high power.

[0153] The vitreous material used for the first lens L1 and the secondlens L2 is the aforementioned LAH53. Between the second lens L2 and theimage surface (recording layer of the optical disc 100) is arranged apolycarbonate protective cover 30. Referring to FIG. 20, S₅ and r₅denote a surface (fifth surface) and the radius of curvature of theprotective cover 30, respectively, while d₄ and d₅ denote the spacing onthe optical axis between the fourth surface S₄ and the fifth surface ofthe second lens L2, and a thickness of protective cover 22,respectively.

[0154] The thickness of the protective cover 30 is preferably not largerthan 0.3 mm. In this first embodiment and the second embodiment, aslater explained, the thickness of the protective cover 30 is set to 0.1mm. The reason is that, if the thickness of the protective cover 30 is0.3 mm or larger, spherical aberration is produced in an amount whichrenders correction difficult, whereas, if the thickness of theprotective cover 30 is less than 0.3 mm, it is possible to suppress thegeneration of spherical aberration.

[0155] The numerical values of the constituent lenses of Example 1 areshown on the following Table 1: TABLE 1 r_(i) d_(i) material type r₁ = ∞d₁ = 1.00 LAH53 r₂ = 12.95 d₂ = 0.20 r₃ = 1.337 d₃ = 1.60 LAH53 r₄ =6.486 d₄ = 0.74 r₅ = ∞ d₅ = 0.1 polycarbonate image surface = ∞

[0156] The cone constant k and fourth to tenth order coefficients A to Dof the compound surface S₂ (diffractive surface S_(2d) and therefractive surface S_(2r)), as the second surface, the third surface S₃and the fourth surface S₄ are indicated in Table 2. In Table 2 and inthe Tables that follow, E denotes an exponential with ten as base. TABLE2 surface k(C₁) A(C₂) B(C₃) C(C₄) D(C₅)  S_(2d)  −3.231E−02 −6.559E−03+5.139E−03 −4.335E−03 +9.493E−04  S_(2r)  −1.000 +7.840E−03 −6.142E−03+5.181E−03 −1.135E−03 S₃  −0.386 −0.356E−02 +0.262E−02 −0.113E−01+0.197E−01 S₄ −16.53 −0.467E−01 +0.355 −1.184 +1.680

[0157]FIGS. 21A, 21B and 21C show the spherical aberration, astigmaticaberration and the distortion aberration of the objective lens 15 of thefirst embodiment, respectively. In the aberration diagrams, shown inFIGS. 21A, 21B and 21C, solid, broken and chain-dotted lines indicatethe values at 405 nm, 403 nm and at 407 nm, respectively. In theaberration diagram of FIG. 21A, thick lines and fine lines indicatevalues on the sagittal image surface and on the tangential imagesurface, respectively. The same applies for similar figures explainedsubsequently. The diffraction reference wavelength is 405 nm, the numberof design order number is N=1, the design wavelength is 405 nm, theincident beam diameter of the laser light is 3.0 mm and the numericalaperture is 0.85.

[0158] Meanwhile, in the respective aberration diagrams of FIGS. 21A to21C, the values at the wavelength of 405 nm, indicated by solid lines,those at a wavelength of 403 nm, indicated by broken lines and those ata wavelength of 407 nm, indicated by chain-dotted lines, substantiallyoverlap one another to render decision almost impossible. This indicatesthat the objective lens 15 in the first embodiment suffers fromchromatic aberration only to an extremely small extent. Thus, it may beseen that, in the present first embodiment, chromatic aberration of theobjective lens 15 may be corrected effectively.

[0159]FIG. 22 shows a lens configuration of the second embodiment of theobjective lens 15 according to the present invention. The first lens L1is formed of a resin material while the second lens 12 is formed of theaforementioned LAH53.

[0160] The objective lens 15 of the second embodiment includes a firstlens set GR1, formed by a first lens L1, as a piano-concave lens ofsynthetic resin, having a compound surface S₂ which is made up by arefractive surface S_(2r) and a diffractive surface S_(2d), constitutedwith the refractive surface S₂, as the base surface, and a second lensset GR2, formed by a second lens L2, as a single large power asphericalglass molded lens. Between the second lens L2 and the image surface(recording layer of the optical disc 100) is arranged a polycarbonateprotective cover 30.

[0161] The numerical values of the constituent lenses of the secondembodiment are indicated in the following Table 3: TABLE 3 r_(i) d_(i)vitreous material r₁ = ∞ d₁ = 1.00 synthetic resin r₂ = 7.650 d₂ = 0.20r₃ = 1.337 d₃ = 1.60 LAH53 r₄ = 6.486 d₄ = 0.74 r₅ = ∞ d₅ = 0.1polycarbonate image surface = ∞

[0162] In the second embodiment, in which the first lens L1 is formed ofsynthetic resin, the objective lens 15 may be reduced in weight.Moreover, since the synthetic resin is less costly than glass, as amaterial, and superior to glass in workability, the objective lens 15can be mas-produced at lower cost.

[0163] Table 4 shows a cone constant k and fourth to tenth orderaspherical coefficients A to D of the compound surface S₂ (diffractivesurface S_(2d) and the refractive surface S_(2r)) as the second surface,third surface S₃ and the fourth surface S₄: TABLE 4 surface k(C₁) A(C₂)B(C₃) C(C₄) D(C₅)  S_(2d)  −3.429E−02 −6.323E−03 +4.451E−03 −4.022E−03+8.960E−04  S_(2r)  −1.000 +1.205E−03 −8.482E−03 +7.666E−03 −1.708E−03S₃  −0.386 −0.356E−02 +0.262E−02 −0.113E−01 +0.197E−01 S₄ −16.53−0.467E−01 +0.355 −1.184 +1.680

[0164]FIGS. 23A, 23B and 23C indicate the spherical aberration,astigmatic aberration and distortion aberration of the objective lens 15of the second embodiment, respectively. The diffraction referencewavelength is 405 nm, the number of design order number is N=1, thedesign wavelength is 405 nm (403 nm to 407 nm) and the numericalaperture is 0.85.

[0165] In the respective aberration diagrams of FIGS. 23A to 23C, thevalues at the wavelength of 405 nm, indicated by solid lines, those at awavelength of 403 nm, indicated by broken lines, and those at awavelength of 407 nm, indicated by chain-dotted lines, substantiallyoverlap one another to render it extremely difficult to discern themfrom one another. This indicates that the objective lens 15 in thesecond embodiment suffers from chromatic aberration only to an extremelysmall extent. Thus, it may be seen that, in the present secondembodiment, chromatic aberration of the objective lens 15 of the secondembodiment has been corrected effectively.

[0166]FIG. 24 shows a lens configuration of the third embodiment of theobjective lens 15 according to the present invention, in which the firstlens L1 is formed by SBSL7 (trade name of a product manufactured byOhara KK) and the second lens L2 is formed by the aforementioned LAH53.

[0167] The objective lens 15 in the third embodiment is comprised of thefirst lens set GR1, formed by a piano-concave lens of glass, having acompound surface S, comprised of a synthetic resin layer 18 formed on arefractive surface S_(2r) as a base surface and a diffractive surfaceS_(2d) formed on the synthetic resin layer 18, and a second lens set GR2formed by a second lens L2, which is a single high-power lense molded ofglass in order to form an aspherical surface. Between the second lens L2and the image surface (recording layer of the optical disc 100) isarranged a polycarbonate protective cover 30.

[0168] The numerical values of the constituent lenses of the secondembodiment are indicated in the following Table 5: TABLE 5 r_(i) d_(i)vitreous material r₁ = ∞ d₁ = 1.00 SBSL7 r_(2r) = 7.750 d₂ = 0.01 r_(2d)= 7.750 d₃ = 0.20 synthetic resin r₃ = 1.337 d₄ = 1.60 LAH53 r₄ = 6.486d₅ = 0.74 r₅ = ∞ d₆ = 0.1 polycarbonate image surface = ∞

[0169] Table 6 shows cone constants k and fourth to tenth orderaspherical coefficients A to D of the compound surface S₂, as the secondsurface, made up by a diffraction surface S_(2d) and a refractivesurface S_(2r), a third surface S₃ and a fourth surface S₄: TABLE 6k(C₁) A(C₂) B(C₃) C(C₄) D(C₅)  S_(2d)  −3.418E−02 −7.456E−03 +5.703E−03−4.609E−03 +9.931E−04  S_(2r)  −1.000 +1.408E−02 −1.077E−02 +8.702E−03−1.875E−03 S₃  −0.386 −0.356E−02 +0.262E−2 −0.113E−01 +0.197E−01 S₄−16.53 −0.467E−01 +0.355 −1.184 +1.680

[0170] In the third embodiment, the diffractive surface S_(2d) is formedon the resin layer 18 layered on the refractive surface S_(2r), becausethe first lens 11 is of SBSL17 which does not permit the compoundsurface to be formed by metal molding. Thus, even if a material whichdoes not permit molding with a metal mold, such as vitreous material, isused for the first lens L1, the refraction-diffraction compound surfacecan be formed by layering the synthetic resin layer 18 on which has beentranscribed the shape of the diffractive surface of the metal mold. Theresult is that the range of selection of the materials used for thefirst lens L1 can be broadened significantly.

[0171]FIGS. 25A, 25B and 25C show the spherical aberration, astigmaticaberration and the distortion aberration of the objective lens 15 of thethird embodiment, respectively. The diffraction reference wavelength,design order number N, design wavelength and the numerical aperture are405 nm, 1, 405 nm and 0.85, respectively.

[0172] In the respective aberration diagrams of FIGS. 25A to 25C, thevalues at the wavelength of 405 nm, indicated by solid lines, those at awavelength of 403 nm, indicated by broken lines, and those at awavelength of 407 nm, indicated by chain-dotted lines, substantiallyoverlap one another to render it difficult to discriminate therespective aberration from one another. This indicates that theobjective lens 15 in the third embodiment suffers from chromaticaberration only to an extremely small extent. Thus, it may be seen that,in the present third embodiment, chromatic aberration of the objectivelens 15 has been corrected effectively.

[0173]FIG. 26 shows a lens configuration of a fourth embodiment of theobjective lens 15 according to the present invention, in which the firstlens L1 and the second lens L2 are formed of a suitable synthetic resinmaterial and LAH53, respectively, and in which the higher orderdiffracted light is used.

[0174] The objective lens 15 in the fourth embodiment is comprised ofthe first lens set GR1, formed by a plano-concave lens of glass, as afirst lens L1, formed of a synthetic resin, and having a compoundsurface S₂ comprised of a diffractive surface S_(2d) layered on a secondsurface or a refractive surface S_(2r) as a base surface, and a secondlens set GR2 formed by a second lens L2, which is a single high-powerlense molded of glass so as to have an aspherical surface. Between thesecond lens L2 and the image surface (recording layer of the opticaldisc 100) is arranged a polycarbonate protective cover 30.

[0175] The numerical values of the constituent lenses of the fourthembodiment are indicated in the following Table 7: TABLE 7 r_(i) d_(i)vitreous material r₁ = ∞ d₁ = 1.00 synthetic resin r₂ = 7.650 d₂ = 0.20r₃ = 1.337 d₃ = 1.60 LAH53 r₄ = 6.486 d₄ = 0.74 r₅ = ∞ d₅ = 0.1polycarbonate image surface = ∞

[0176] In the fourth embodiment, the first lens L1 is formed ofsynthetic resin, so that the objective lens 15 can be reduced in weight.Moreover, since the synthetic resin is less costly than glass, asmaterial, and is superior in workability, the objective lens 15 can bemass-produced at low cost.

[0177] Table 8 shows cone constants k and fourth to tenth orderaspherical coefficients A to D of the compound surface S₂, as the secondsurface, made up by a diffraction surface S_(2d) and a refractivesurface S_(2r), a third surface S₃ and a fourth surface S₄: TABLE 8k(C₁) A(C₂) B(C₃) C(C₄) D(C₅)  S_(2d)  −3.429E−02 −6.352E−04 +4.464E−04−4.013E−04 +8.916E−05  S_(2r)  −1.000 +1.211E−02 −8.507E−03 +7.647E−03−1.699E−03 S₃  −0.386 −0.356E−02 +0.262E−02 −0.113E−01 +0.197E−01 S₄−16.53 −0.467E−01 +0.355 −1.184 +1.680

[0178]FIGS. 27A, 27B and 27C show the spherical aberration, astigmaticaberration and the distortion aberration of the objective lens 15 of thefourth embodiment, respectively. The diffraction reference wavelength,design order number N, design wavelength and the numerical aperture are405 nm, 10, 405 nm and 0.85, respectively.

[0179] In the respective aberration diagrams of FIGS. 27A to 27C, thevalues at the wavelength of 405 nm, indicated by solid lines, those at awavelength of 403 nm, indicated by broken lines, and those at awavelength of 407 nm, indicated by chain-dotted lines, substantiallyoverlap one another to render it difficult to discriminate therespective aberrations from one another. This indicates that theobjective lens 15 in the fourth embodiment suffers from chromaticaberration only to an extremely small extent. Thus, it may be seen that,in the present fourth embodiment, chromatic aberration of the objectivelens 15 has been corrected effectively.

[0180] By changing the design of the objective lens to the designemploying the diffracted light of the higher order as the objective lens15 of the fourth embodiment, the height and the width of the staircaseshape of the diffraction surface S_(2d) are the order number times thosefor the first order diffracted light. Thus, in case the width of thestaircase shape of the rim portion of the lens is smaller than that ofthe other lens portion, the designing employing the larger order numberis effective from the perspective of workability of the staircase shapeof the diffraction surface S_(2d).

[0181]FIG. 28 shows the lens structure of the fifth embodiment of theobjective lens 15 according to the present invention. Specifically, thefirst lens L1 is formed of a suitable synthetic resin material, thesecond lens L2 is formed of LAH53 and the separation between the firstlens set GR1 and the second lens set GR2 is set so as to be not lessthan 1 mm.

[0182] The objective lens 15 in the fifth embodiment is comprised of thefirst lens set GR1, formed by a piano-concave lens, as a first lens L1,formed of a synthetic resin, and having a compound surface S₂ comprisedof a diffractive surface S_(2d) layered on a refractive surface S_(2r)as a base surface, and a second lens set GR2 formed by a second lens L2,which is a single high-power lense molded of glass so as to have anaspherical surface. Between the second lens L2 and the image surface(recording layer of the optical disc 100) is arranged a polycarbonateprotective cover 30.

[0183] The numerical values of the constituent lenses of the fifthembodiment are indicated in the following Table 9: TABLE 9 r_(i) d_(i)vitreous material r₁ = ∞ d₁ = 1.00 synthetic resin r₂ = 7.650 d₂ = 3.00r₃ = 1.337 d₃ = 1.60 LAH53 r₄ = 6.486 d₄ = 0.74 r₅ = ∞ d₅ = 0.1polycarbonate image surface = ∞

[0184] In the fifth embodiment, the first lens L1 is formed of syntheticresin, so that the objective lens 15 can be reduced in weight. Moreover,since the synthetic resin is less costly than glass, as material, and issuperior in workability, the objective lens 15 can be mass-produced atlow cost.

[0185] Table 10 shows cone constants k and fourth to tenth orderaspherical coefficients A to D of the compound surface S₂, as secondsurface, made up by a diffraction surface S_(2d) and a refractivesurface S_(2r), a third surface S₃ and a fourth surface S₄: TABLE 10k(C₁) A(C₂) B(C₃) C(C₄) D(C₅)  S_(2d)  −3.429E−02 −9.168E−03 +1.209E−02−1.006E−02 +2.339E−03  S_(2r)  −1.000 +1.747E−02 −2.304E−02 +1.918E−02−4.457E−03 S₃  −0.386 −0.356E−02 +0.262E−02 −0.113E−01 +0.197E−01 S₄−16.53 −0.467E−01 +0.355 −1.184 +1.680

[0186]FIGS. 29A, 29B and 29C show the spherical aberration, astigmaticaberration and the distortion aberration of the objective lens 15 of thefifth embodiment, respectively. The diffraction reference wavelength,design order number N, design wavelength and the numerical aperture are405 nm, 1, 405 nm and 0.85, respectively.

[0187] In the respective aberration diagrams of FIGS. 29A to 29C, thevalues at the wavelength of 405 nm, indicated by solid lines, those at awavelength of 403 nm, indicated by broken lines, and those at awavelength of 407 nm, indicated by chain-dotted lines, substantiallyoverlap one another to render it difficult to discriminate therespective aberrations from one another. This indicates that theobjective lens 15 in the fifth embodiment suffers from chromaticaberration only to an extremely small extent. Thus, it may be seen that,in the present fifth embodiment, chromatic aberration of the objectivelens 15 has been corrected effectively.

[0188] Meanwhile, when the objective lens 15 according to the presentinvention is assembled to the optical pickup 12 and put to use in thisstate, the objective lens is driven by a biaxial actuator inasmuch asfocusing servo and tracking servo need to be applied. On application ofthe focusing servo and tracking servo, the objective lens 15 tends to besubjected to resonant oscillations.

[0189] With the objective lens 15 of the fifth embodiment, the spacingd₂ between the first lens set GR1 and the second lens set GR2 is set to3.0 mm. It should be noted that such designing which will cancel out theresonant oscillations on application of the focusing servo and trackingservo is enabled by adjusting the spacing d₂ to shift the position ofcenter of gravity of the objective lens 15, which is the lens combinedfrom the first lens L1 and the second lens L2, to cancel out theresonant oscillations produced on application of the focusing servo andthe tracking servo.

[0190] As described above, the objective lens 15 according to thepresent invention is a hybrid double lens set typerefraction-diffraction lens comprised of the first lens set GR1 formedby the first lens L1 and the second lens set GR2 formed by the singlelens L2 with an aspherical surface. The first lens L1 has the compoundsurface S₂ comprised of the phase diffraction surface S_(2d) formed onthe aspherical refraction surface S_(2r). The refractive surface (basesurface) S_(2r), to which is added the refractive surface S_(2d), isformed as an aspherical concave surface. This designing allows to setthe on-axis chromatic aberration to be approximately equal to zero evenif the wavelength of the laser light from the laser light emittingelement 16 is changed, so that the working distance may be designed to alarger value to reduce the radius of curvature of the base surface, asthe necessary numerical aperture (NA) is maintained, thus providing forfacilitated machining of the phase diffraction grating forming thecompound surface S₂.

[0191] In an objective lens used for an optical pickup, adapted forcoping with an optical disc having a high information recordingcapacity, it is requested to set the chromatic aberration to 0.05 μm/nmor less. In the conventional single type objective lens 201, shown inFIG. 1, the chromatic aberration on the order of ±0.6 μm/nm is producedfor wavelength variations of +2 nm of the laser light, whereas, in eachobjective lens according to the present invention, described above, thechromatic aberration can be low and on the order of 0.01 μm/nm forwavelength variations on the order of ±2 nm, as conventionally, so that,in the optical pickup and in the optical disc device, informationrecording and/or reproduction can be achieved in stability and the laserlight spot diameter can be diminished to close to the diffractionthreshold. As a consequence, the objective lenses of the presentinvention can be of sufficient performance for coping with the opticaldisc of such standard in which the information recording density hasbeen raised by narrowing the track pitch.

[0192] By employing the objective lens of the present invention for anoptical pickup including means for increasing or varying the laser powerfor coping with the rewritable optical disc for reducing the lasernoise, it is possible to improve the reproducing performance and therecording performance of the high recording density information.

[0193] Moreover, by employing the optical pickup 12 employing theobjective lens of the present invention, it is possible to furnish anoptical disc device having improved recording and/or reproductionperformance for the high recording density information.

[0194] The particular shape or configuration of respective portions ofthe embodiments of the present invention has been disclosed in theperspective of illustration and hence the scope of the present inventionshould be defined only in light of the claims without being construed ina limiting fashion.

INDUSTRIAL APPLICABILITY

[0195] With the objective lens and an optical pickup employing theobjective lens, according to the present invention, the chromaticaberration for the light beam with a wavelength within not larger than afew nm about 420 nm or less can be effectively corrected to narrow downthe spot diameter of the light beam to close to the diffractionthreshold, so that, by reducing the track pitch of the recording track,it is possible to cope with standard of the optical recording medium inwhich the track pitch of the recording track is narrowed to raise theinformation recording density.

[0196] With the optical disc device including the optical pickupemploying the objective lens according to the present invention, theinformation can be recorded to high density on the optical recordingmedium in association with the optical recording medium which hasenabled the high density recording, while the information recorded tohigh density can be read out from the optical recording medium.

1. (Amended) An objective lens for an optical pickup, constituted of afirst lens set including a diffractive type lens and a second sens setincluding a refractive type lens set in order from an object side, andhaving a numerical aperture not less than 0.8, and adapted forcorrecting the chromatic aberration at an image point on an optical axisfor light with a wavelength within several nm about a referencewavelength which is not larger than 420 nm, wherein said first lens sethas a compound surface constituted by adding a diffractive surfacehaving a positive refractive power to an aspherical refractive surfacehaving a negative refractive power; an amount of sag of the asphericalsurface of said first lens set having a negative refractive power isdescribed by a polynominal of an even order number with respect to aradius, as the cone coefficient of the aspherical coefficient (k) is setto −1; an amount of sag of the diffractive surface of the first lens setis described by a polynominal of an even order number with respect to aradius; the order number of the polynominal of the aspherical surface isequal to the order number of the polynominal of the diffractive surface;the coefficients of the same order numbers of the polynominal of the sagof the aspherical surface are equated to those of the polynominal of thesag of the diffractive surface in such a manner as to meet theequations: k=−1 C ₁=(N−1)c/2 C ₂=(N−1)A C ₃=(N−1)C C ₄=(N−1)D  where C₁,C₂, C₃, C₄, . . . are coefficients of respective order numbers of thepolynominal of said aspherical surface, c/2 is the second ordercoefficient of the polynominal of said diffractive surface and A, B, C,D, . . . are coefficients of the respective orders of the polynominal ofsaid diffractive surface; the lens forming said second lens set isformed by a single lens including at least one aspherical surface; andan aperture is provided between said first and second lens sets. 2.(Amended) The objective lens for an optical pickup according to claim 1wherein the aperture provided between said first and second lens sets isformed by a thin film of e.g., metal provided on the surface of arefractive type lens of the second lens set forming said second lensset.
 3. (Amended) An objective lens for an optical pickup, constitutedof a first lens set including a diffractive type lens and a second sensset including a refractive type lens set in order from an object side,and having a numerical aperture not less than 0.8, and adapted forcorrecting the chromatic aberration at an image point on an optical axisfor light with a wavelength within several nm about a referencewavelength which is not larger than 420 nm, wherein said first lens sethas a compound surface constituted by adding a diffractive surfacehaving a positive refractive power to an aspherical refractive surfacehaving a negative refractive power; an amount of sag of the asphericalsurface of said first lens set having a negative refractive power isdescribed by a polynominal of an even order number with respect to aradius, as the cone coefficient of the aspherical coefficient (k) is setto −1; an amount of sag of the diffractive surface of the first lens setis described by a polynominal of an even order number with respect to aradius; the order number of the polynominal of the aspherical surface isequal to the order number of the polynominal of the diffractive surface;the coefficients of the same order numbers of the polynominal of the sagof the aspherical surface are equated to those of the polynominal of thesag of the diffractive surface in such a manner as to meet theequations: k=−1 C ₁=(N−1)c/2 C ₂=(N−1)A C ₃=(N−1)C C ₄=(N−1)D  where C₁,C₂, C₃, C₄, . . . are coefficients of respective order numbers of thepolynominal of said aspherical surface, c/2 is the second ordercoefficient of the polynominal of said diffractive surface and A, B, C,D, . . . are coefficients of the respective orders of the polynominal ofsaid diffractive surface; the lens forming said second lens set isformed by a single lens including at least one aspherical surface; andconcentric irregularities finer than irregularities of said diffractivesurface are formed on the compound surface of said first lens set, saidconcentric irregularities being of a periodic structure having a periodequal to about one half said reference wavelength and having anamplitude equal to approximately one fourth the reference wavelength. 4.(Amended) An optical pickup comprising a laser light radiating element,radiating the laser light, an objective lens for converging the laserlight on a recording layer of an optical recording medium, a lightreceiving element for receiving the laser light and an optical elementfor causing the laser light radiated from said laser light radiatingelement to fall on said objective lens and for causing the laser lightreflected by the recording layer of the optical recording medium andtransmitted through said objective lens to fall on said light receivingelement; said objective lens is constituted of a first lens setincluding a diffractive type lens and a second sens set including arefractive type lens set in order from an object side, and having anumerical aperture not less than 0.8, and adapted for correcting thechromatic aberration at an image point on an optical axis for light witha wavelength within several nm about a reference wavelength which is notlarger than 420 nm, wherein said first lens set has a compound surfaceconstituted by adding a diffractive surface having a positive refractivepower to an aspherical refractive surface having a negative refractivepower; an amount of sag of the aspherical surface of said first lens sethaving a negative refractive power is described by a polynominal of aneven order number with respect to a radius, as the cone coefficient ofthe aspherical coefficient (k) is set to −1; an amount of sag of thediffractive surface of the first lens set is described by a polynominalof an even order number with respect to a radius; the order number ofthe polynominal of the aspherical surface is equal to the order numberof the polynominal of the diffractive surface; the coefficients of thesame order numbers of the polynominal of the sag of the asphericalsurface are equated to those of the polynominal of the sag of thediffractive surface in such a manner as to meet the equations: k=−1 C₁=(N−1)c/2 C ₂=(N−1)A C ₃=(N−1)C C ₄=(N−1)D  where C₁, C₂, C₃C₄, . . .are coefficients of respective order numbers of the polynominal of saidaspherical surface, c/2 is the second order coefficient of thepolynominal of said diffractive surface and A, B, C, D, . . . arecoefficients of the respective orders of the polynominal of saiddiffractive surface; the lens forming said second lens set is formed bya single lens including at least one aspherical surface; and an apertureis provided between said first and second lens sets.
 5. (Amended) Theoptical pickup according to claim 4 wherein concentric irregularitiesfiner than irregularities of said diffractive surface are formed on thecompound surface of said first lens set, said concentric irregularitiesbeing of a periodic structure having a period equal to about one halfsaid reference wavelength and having an amplitude equal to approximatelyone fourth the reference wavelength.
 6. (Amended) The optical pickupaccording to claim 4 wherein the cross-sectional shape of thediffractive surface of said first lens set is a staircase shape havingstep differences affording phase differences equal to integer multiplesof a design wavelength.
 7. (Amended) The optical pickup according toclaim 4 wherein the working distance from the trailing lens surface tosaid image point is not less than 0.5 mm.
 8. (Amended) The opticalpickup according to claim 4 wherein respective lenses forming said firstand second lens sets are arranged in a common lens barrel.
 9. (Amended)The optical pickup according to claim 4 wherein said first lens set hasa surface closest to a light source side as a planar surface and has arefractive power for the design wavelength equal to zero.
 10. (Amended)The optical pickup according to claim 9 wherein the aperture providedbetween said first and second lens sets is formed by a thin film ofe.g., metal provided on the surface of a refractive type lens of thesecond lens set forming said second lens set.
 11. (Amended) The opticalpickup according to claim 4 wherein the lens forming said first lens setis formed of synthetic resin.
 12. (Amended) The optical pickup accordingto claim 4 wherein a protective cover having a thickness not larger than0.3 mm is arranged between said second lens set and the image surfaceand wherein the spherical aberration attributable to said protectivecover is corrected.
 13. (Amended) The optical pickup according to claim4 wherein the lens forming said first lens set and the lens barrel areformed as one from synthetic resin.
 14. (Amended) The optical pickupaccording to claim 13 wherein a lens forming said second lens set isarranged in said lens barrel formed as one with the lens forming thefirst lens set.
 15. (Amended) An optical pickup comprising a laser lightradiating element, radiating the laser light, an objective lens forconverging the laser light on a recording layer of an optical recordingmedium, a light receiving element for receiving the laser light and anoptical element for causing the laser light radiated from said laserlight radiating element to fall on said objective lens and for causingthe laser light reflected by the recording layer of the opticalrecording medium and transmitted through said objective lens to fall onsaid light receiving element; said objective lens is constituted of afirst lens set including a diffractive type lens and a second sens setincluding a refractive type lens set in order from an object side, andhaving a numerical aperture not less than 0.8, and adapted forcorrecting the chromatic aberration at an image point on an optical axisfor light with a wavelength within several nm about a referencewavelength which is not larger than 420 nm, wherein said first lens sethas a compound surface constituted by adding a diffractive surfacehaving a positive refractive power to an aspherical refractive surfacehaving a negative refractive power; an amount of sag of the asphericalsurface of said first lens set having a negative refractive power isdescribed by a polynominal of an even order number with respect to aradius, as the cone coefficient of the aspherical coefficient (k) is setto −1; an amount of sag of the diffractive surface of the first lens setis described by a polynominal of an even order number with respect to aradius; the order number of the polynominal of the aspherical surface isequal to the order number of the polynominal of the diffractive surface;the coefficients of the same order numbers of the polynominal of the sagof the aspherical surface are equated to those of the polynominal of thesag of the diffractive surface in such a manner as to meet theequations: k=−1 C₁=(N−1)c/2 C ₂=(N−1)A C ₃=(N−1)C C ₄=(N−1)D  where C₁,C₂, C₃, C₄, . . . are coefficients of respective order numbers of thepolynominal of said aspherical surface, c/2 is the second ordercoefficient of the polynominal of said diffractive surface and A, B, C,D, . . . are coefficients of the respective orders of the polynominal ofsaid diffractive surface; the lens forming said second lens set isformed by a single lens including at least one aspherical surface; andconcentric irregularities finer than irregularities of said diffractivesurface are formed on the compound surface of said first lens set, saidconcentric irregularities being of a periodic structure having a periodequal to about one half said reference wavelength and having anamplitude equal to approximately one fourth the reference wavelength.16. (Amended) The optical pickup according to claim 15 wherein thecross-sectional shape of the diffractive surface of said first lens setis a staircase shape having step differences affording phase differencesequal to integer multiples of a design wavelength.
 17. (Amended) Theoptical pickup according to claim 15 wherein the working distance fromthe trailing lens surface to said image point is not less than 0.5 mm.18. (Amended) The optical pickup according to claim 15 whereinrespective lenses forming said first and second lens sets are arrangedin a common lens barrel.
 19. (Amended) The optical pickup according toclaim 15 wherein an aperture is provided between said first and secondlens sets.
 20. (Amended) The optical pickup according to claim 15wherein said first lens set has a surface closest to a light source sideas a planar surface and has a refractive power for the design wavelengthequal to zero.
 21. (Amended) The optical pickup according to claim 19wherein the aperture provided between said first and second lens sets isformed by a thin film of e.g., metal provided on the surface of arefractive type lens of the second lens set forming said second lensset.
 22. (Amended) The optical pickup according to claim 15 wherein thelens forming said first lens set is formed of synthetic resin. 23.(Amended) The optical pickup according to claim 15 wherein a protectivecover having a thickness not larger than 0.3 mm is arranged between saidsecond lens set and the image surface and wherein the sphericalaberration attributable to said protective cover is corrected. 24.(Amended) The optical pickup according to claim 15 wherein the lensforming said first lens set and the lens barrel are formed as one fromsynthetic resin.
 25. (Amended) The optical pickup according to claim 24wherein a lens forming said second lens set is arranged in said lensbarrel formed as one with the lens forming the first lens set. 26.(Amended) An optical disc device for recording and/or reproducing theinformation for recording and/or reproducing the information for arotating disc-shaped recording medium by an optical pickup movable alongthe radius of said disc-shaped recording medium; said optical pickupincluding a laser light radiating element, radiating the laser light ofa wavelength equal to 420 nm or less, an objective lens for convergingthe laser light on a recording layer of an optical recording medium, alight receiving element for receiving the laser light and an opticalelement for causing the laser light radiated from said laser lightradiating element to fall on said objective lens and for causing thelaser light reflected by the recording layer of the optical recordingmedium and transmitted through said objective lens to fall on said lightreceiving element; said objective lens is constituted of a first lensset including a diffractive type lens and a second sens set including arefractive type lens set in order from an object side, and having anumerical aperture not less than 0.8, and adapted for correcting thechromatic aberration at an image point on an optical axis for light witha wavelength within several nm about a reference wavelength which is notlarger than 420 nm, wherein said first lens set has a compound surfaceconstituted by adding a diffractive surface having a positive refractivepower to an aspherical refractive surface having a negative refractivepower; an amount of sag of the aspherical surface of said first lens sethaving a negative refractive power is described by a polynominal of aneven order number with respect to a radius, as the cone coefficient ofthe aspherical coefficient (k) is set to −1; an amount of sag of thediffractive surface of the first lens set is described by a polynominalof an even order number with respect to a radius; the order number ofthe polynominal of the aspherical surface is equal to the order numberof the polynominal of the diffractive surface; the coefficients of thesame order numbers of the polynominal of the sag of the asphericalsurface are equated to those of the polynominal of the sag of thediffractive surface in such a manner as to meet the equations: k=−1 C₁=(N−1)c/2 C ₂=(N−1)A C₃=(N−1)C C ₄=(N−1)D  where C₁, C₂, C₃, C₄, . . .are coefficients of respective order numbers of the polynominal of saidaspherical surface, c/2 is the second order coefficient of thepolynominal of said diffractive surface and A, B, C, D, . . . arecoefficients of the respective orders of the polynominal of saiddiffractive surface; the lens forming said second lens set is formed bya single lens including at least one aspherical surface; and an apertureis provided between said first and second lens sets.
 27. (Amended) Anoptical disc device for recording and/or reproducing the information forrecording and/or reproducing the information for a rotating disc-shapedrecording medium by an optical pickup movable along the radius of saiddisc-shaped recording medium; said optical pickup including a laserlight radiating element, radiating the laser light of a wavelength equalto 420 nm or less, an objective lens for converging the laser light on arecording layer of an optical recording medium, a light receivingelement for receiving the laser light and an optical element for causingthe laser light radiated from said laser light radiating element to fallon said objective lens and for causing the laser light reflected by therecording layer of the optical recording medium and transmitted throughsaid objective lens to fall on said light receiving element; saidobjective lens is constituted of a first lens set including adiffractive type lens and a second sens set including a refractive typelens set in order from an object side, and having a numerical aperturenot less than 0.8, and adapted for correcting the chromatic aberrationat an image point on an optical axis for light with a wavelength withinseveral nm about a reference wavelength which is not larger than 420 nm,wherein said first lens set has a compound surface constituted by addinga diffractive surface having a positive refractive power to anaspherical refractive surface having a negative refractive power; anamount of sag of the aspherical surface of said first lens set having anegative refractive power is described by a polynominal of an even ordernumber with respect to a radius, as the cone coefficient of theaspherical coefficient (k) is set to −1; an amount of sag of thediffractive surface of the first lens set is described by a polynominalof an even order number with respect to a radius; the order number ofthe polynominal of the aspherical surface is equal to the order numberof the polynominal of the diffractive surface; the coefficients of thesame order numbers of the polynominal of the sag of the asphericalsurface are equated to those of the polynominal of the sag of thediffractive surface in such a manner as to meet the equations: k=−1 C₁=(N−1)c/2 C ₂=(N−1)A C ₃=(N−1)C C ₄=(N−1)D  where C₁, C₂, C₃, C₄, . . .are coefficients of respective order numbers of the polynominal of saidaspherical surface, c/2 is the second order coefficient of thepolynominal of said diffractive surface and A, B, C, D, . . . arecoefficients of the respective orders of the polynominal of saiddiffractive surface; the lens forming said second lens set is formed bya single lens including at least one aspherical surface; and concentricirregularities finer than irregularities of said diffractive surface areformed on the compound surface of said first lens set, said concentricirregularities being of a periodic structure having a period equal toabout one half said reference wavelength and having an amplitude equalto approximately one fourth the reference wavelength.
 28. (Deleted) 29.(Deleted)